Are you having a laugh?
When someone asks you this, it is always best not to actually laugh.
There is likely to be very little humour involved in the situation.
“Are you having a laugh?” is just a slightly heated and informal way of saying: “I whole heartedly disagree with that statement and think that the only plausible reason why you could have said it, is that you are trying to make a joke.”
“That’ll be twenty pounds please.” “Twenty quid for a bottle of shampoo? Are you having a laugh? “
“£500 to cut down a tree - you're having a laugh, mate!”
"Are you having a laugh?" is similar to: "Are you being serious?" or "Are you kidding me?"